Material analysis based on imaging effective atomic numbers

ABSTRACT

Effective atomic numbers associated with pixels in a region are received. An effective atomic number is associated with each pixel in the region. X-ray data for the region is received, and an item within the region is identified from the x-ray data. Some of the pixels in the region are correlated with the item such that the item is associated with an effective atomic number. An image of the region is rendered. The pixels of the item have a display style based on the effective atomic number of the item.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 60/894,615, titled IMAGING EFFECTIVE ATOMIC NUMBER OF MATERIALS FORTHREAT DETECTION, and filed on Mar. 13, 2007, which is incorporated byreference in its entirety.

TECHNICAL FIELD

This description relates to analyzing materials based on imagingeffective atomic numbers of the materials.

BACKGROUND

Color-coded images representing an average atomic number of objects ofinterest such as baggage and cargo and items included in these objectsmay be presented to an operator. The images of the average atomic numbergenerally do not account for the effects of an overlaying or underlayingmaterial around the object of interest.

SUMMARY

In one general aspect, effective atomic numbers associated with pixelsin a region are received. An effective atomic number is associated witheach pixel in the region. X-ray data for the region is received, and anitem within the region is identified from the x-ray data. Some of thepixels in the region are correlated with the item such that the item isassociated with an effective atomic number. An image of the region isrendered. The pixels of the item have a display style based on theeffective atomic number of the item.

Implementations may include one or more of the following features. Anindication of a range of effective atomic numbers of interest may bereceived. The display style may include color, and a color may beassociated with the range of effective atomic numbers of interest. Thedisplay style may include one or more patterns, and a pattern isassociated with the range of effective atomic numbers of interest.Additional items within the region may be identified from the x-raydata. The additional items may cover the item in a region of overlap,and the additional items may have a effective atomic numbers that aredifferent than the effective atomic number of the item. The effectiveatomic numbers of the additional items may be within the range ofeffective atomic numbers of interest, and the additional items and theregion of overlap may be represented by the color associated with therange of effective atomic numbers of interest.

In some implementations, receiving an indication of a range of effectiveatomic numbers of interest may include accessing a predetermined rangeof effective atomic numbers of interest. A re-sizeable selector thatallows selection of one or more ranges of effective atomic numbers ofinterest may be displayed. The selector may be movable. A size of theselector may correspond to one or more ranges of effective atomicnumbers of interest, and a position of the selector may correspond to amean of the effective atomic numbers included in the range. Identifyingan item within the region from the x-ray data may include determiningedges within the x-ray data and identifying the item based on the edges.Identifying an item within the region from the x-ray data may includeaggregating pixels having similar attenuation values and identifying anitem based on one or more boundaries around the aggregated pixels.

In another general implementation, a system of imaging effective atomicnumbers includes a processor configured to receive effective atomicnumbers associated with pixels in a region. An effective atomic numberis associated with each pixel in the region. X-ray data is received forthe region, an item is identified within the region from the x-ray data,and some of the pixels in the region are correlated with the item suchthat the item is associated with an effective atomic number. The systemincludes a display configured to render an image of the region, wherethe pixels of the item have a display style based on the effectiveatomic number of the item.

Implementations may include one or more of the following features. Thesystem may include a source configured to expose an area imaged by thepixels to x-rays of at least two energies. The system may include asensor to detect x-rays of at least two energies. A filter may bepositioned between the area and the sensor, and the filter may separatethe x-rays of at least two energies. The display may be a touch screen.An opening may receive an object. The opening may receive cargo and theobject may be luggage.

In another general implementation, effective atomic numbers associatedwith pixels in a region are received. An effective atomic number isassociated with each pixel in the region. Penetrating spectral data forthe region is received, and an item within the region is identified fromthe penetrating spectral data. Some of the pixels in the region arecorrelated with the item such that the item is associated with aneffective atomic number. An image of the region is rendered. The pixelsof the item have a display style based on the effective atomic number ofthe item.

Implementations may include one or more of the following features. Thepenetrating spectral data may include data based on x-rays of at leasttwo energy levels.

Implementations of any of the techniques described above may include amethod, a process, a system, a device, an apparatus, or instructionsstored on a computer-readable medium. The details of one or moreimplementations are set forth in the accompanying drawings and thedescription below. Other features will be apparent from the descriptionand drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a materials analysis system.

FIGS. 2A and 2B show an example interface for interacting with an imagerepresenting effective atomic numbers.

FIGS. 2C-2I show illustrations of image of items that are color-codedbased on effective atomic numbers of the items is shown.

FIG. 3 shows an example effective atomic number imaging system.

FIG. 4 shows an example system for imaging effective atomic numbers.

FIG. 5 shows an example process for imaging effective atomic numbers ofmaterials.

FIGS. 6A-6C show example images of materials in effective atomic numberspace.

DETAILED DESCRIPTION

Referring to FIG. 1, an illustration of an example materials analysissystem 100 is shown. The system 100 may be, for example, a threatdetection system used to determine whether an object of interest (suchas a suitcase 110) holds a hazardous, or potentially hazardous,material. For example, the system 100 may be used to determine whetherthe suitcase 110 holds weapons (such as guns or knives), potentiallydangerous items (such as scissors or needles), explosives, or plasticpipe bombs. The system 100 displays an image of the object of interestand items included within the object of interest on an effective atomicnumber display 130. As discussed in more detail below, the imagedisplayed on the display 130 provides visual separation anddifferentiation of items based on the effective atomic number of theitems, regardless of whether the items overlay or underlay other itemshaving different effective atomic numbers. The image displayed on thedisplay may provide a more accurate representation of the items includedin the object of interest as compared to techniques that display anaverage atomic number of the items and, thus, an image based oneffective atomic numbers may allow more accurate identification of theitems.

One approach to providing color imaging of cargo and hand carry baggageemploys organic-inorganic type imaging with color overlays identifyingtargeted materials. This type of imaging is generally based on theaverage atomic number (Z) of the materials without removing the effectsof overlying or underlying materials. As such, in many cases, thedisplay of the average atomic number of the materials presentsinaccurate information to an operator. For example, an orange color maybe associated with an organic material, such as an explosive. However,if the explosive material is covered by a higher atomic number material,such as a sheet of metal, the explosive no longer shows on the displayas an orange organic object. Instead, the explosive is shown with acolor associated with the atomic number of the metal sheet of metal. Asa result, the display of the average atomic number gives the operator afalse impression that the explosive material is not a potential threat.One technique to mitigate this effect is the use of “operator assist”techniques in which, for example, red bounding boxes (which may becalled threat boxes) may be used to identify a targeted material.However, display of red bounding boxes may be disfavored because suchtechniques are believed to distract the operator from the function offinding weapons. In particular, the red bounding boxes may confuse anddistract the operator. In general, the overall performance of such“operator assist” techniques results in relatively low detection andhigh false alarm rates. Detection rate is generally the number ofcorrect detections (e.g., a determination that a hazardous item ispresent when a hazardous item is in fact present) made over a period oftime, and false alarm rate is the number of incorrect detections (e.g.,a determination that a hazardous item is present when a hazardous itemis not actually present) made over a period of time.

However, the display 130 displays an image in a true effective atomicnumber color space as opposed to an average atomic number color space.In other words, the image shown by the display 130 may be used todifferentiate and separate items according to effective atomic number.Such an image may provide the operator with a more accuraterepresentation of the suitcase 110 and the contents of the suitcase 110.

The system 100 includes a screener apparatus 120 through which thesuitcase 120 passes. The screener 120 exposes the suitcase 110 to x-rayradiation having at least two energy levels, and the screener 120 sensesx-ray radiation that passes through the suitcase 110. The effectiveatomic number of the materials of the suitcase 110 and the materials ofany items within the suitcase 110 may be determined by exposing thematerial to x-rays of two of more different energies and analyzing theabsorption of the x-rays by the materials at the different energies. Theeffective atomic number is a material-specific property, and theeffective atomic number may be referred to as the true Z number.Briefly, materials that readily absorb x-rays, such as metals, tend tohave relatively high effective atomic numbers (e.g., above 20). Thesematerials absorb both lower-energy x-rays and higher-energy x-rays.Materials that absorb x-rays less readily, such as organic materials andplastics, tend to have lower effective atomic numbers (e.g., between 5and 12). These materials tend to absorb fewer low energy x-rays ascompared to materials having a higher effective atomic number. Organicmaterials may include items such as food and clothing, and inorganicmaterials may include items made from materials such as metal Inparticular, characteristic differences in photoelectric effectscattering and Compton scattering, and comparison of the pairwisedifferential attenuation of the higher energy x-rays and the lowerenergy x-rays may be used to determine whether differences inattenuation may be attributed to the presence of a specific materialregardless of whether the material is overlayed (e.g., covered) orunderlayed by a different material of a different atomic number.Techniques for determining the effective atomic number may be found incommonly owned U.S. Pat. Nos. 5,319,547, 5,600,700, 5,642,393, and6,088,423, all of which are hereby incorporated by reference in itsentirety.

The items in the image shown on the display 130 are each associated withone or more display styles. Each display style indicates an effectiveatomic number of a material from which the items are composed. Thedisplay style may be a particular color. For example, items having aneffective atomic number between five and ten, such items includingorganic material, may be colored orange, and items having an effectiveatomic number between seventeen and twenty-two, such as items includingmetallic material, may be colored blue. Items within the suitcase 110may overlay or underlay each other. For example, the suitcase 110 mayinclude a first item having an effective atomic number of five and asecond item having an effective atomic number of twenty-one. The firstitem may overlay the second item if the first item covers, or partiallycovers, the second item. The first item may underlay an item if thefirst item is covered, or partially covered, by the second item. Aregion of overlap occurs at the overlay or the underlay. Rather thandisplaying the average atomic number of the first and second item at theregion of overlap, the display 130 may display an image in which thefirst and second item with the region of overlap colored based on areceived atomic number of interest or a range of atomic numbers ofinterest. In some implementations, the display style may include shadingand/or cross-hatching in addition to, or instead of, color. In someimplementations, the display style may include a texture.

In some implementations, the display 130 also may display a selector 150that allows a selection of one or more ranges of effective atomicnumbers to include in the color-coded image shown on the display 130. Inthe example shown, a plastic pipe bomb 112 is covered by a sheet ofmetal 114. However, the color-coded image shown on the display 130 showsonly the plastic pipe bomb 112 because the setting of the selector 150specifies that materials having effective atomic numbers similar to thatof the sheet of metal 114 be excluded from the color-coded image. Insome implementations, materials having effective atomic numbers similarto that of the sheet of metal 114 are not excluded from the color-codedimage. For example, both the sheet of metal 114 and the plastic pipebomb 112 are shown, and any overlap region between the sheet of metal114 and the plastic pipe bomb 112 is colored according to an indicatedeffective atomic number of interest. Although specific examples arediscussed below, the effective atomic number display 130 may displayeffective atomic number information derived from any type of dual energyx-ray system

Additionally, as shown in FIGS. 2A and 2B, in some implementations, aninterface shown on the display 130 may allow the operator to select oneor more ranges of effective atomic numbers to include in the image.Thus, selecting a range may allow the operator to view only items thathave effective atomic numbers associated with explosives. In someimplementations, selecting a range of effective atomic numbers mayresult in items that have effective atomic numbers associated withexplosives moved to the foreground, or otherwise emphasized, but otheritems with different effective atomic numbers are still shown in theimage. As a result, the bounding boxes discussed above may not benecessary because the operator may be able to discern objects based onthe more accurate representation of the objects themselves in the imagewithout using operator assist techniques (such as red bounding boxesdrawn around potential objects of interest) to draw the operator'sattention to the object. Displaying the image in true effective atomicnumber space may result in a higher probability of detection and a lowerfalse alarm rate as compared to implementations that use red boundingboxes to highlight potential objects of interest.

In addition to providing an image based on effective atomic number, thedisplay 130 also allows a user to select one or more ranges of effectiveatomic numbers to include in the displayed image. The one or more rangesof effective atomic numbers may be selected using the selector 150. Forexample, an operator of the system 100 may select the range of effectiveatomic numbers using the selector 150. In some implementations, theranges are preset ranges that may be set by, for example, themanufacturer. In some implementations, the ranges are set by asupervisor and cannot be modified by others.

As discussed in more detail below, selection of the ranges allows anitem made of a particular material with an effective atomic numberwithin the range to be shown on the foreground of the image even if theitem is covered by, or rests on top of, items composed of anothermaterial. For example, a plastic explosive 112 enclosed in the suitcase110 and covered by a sheet of metal 114 appears without being obstructedby the sheet of metal 114 when the selection of the range of effectiveatomic numbers to include in the image includes the effective atomicnumber of the plastic explosive 112 but the selected range does notinclude the effective atomic number of the sheet of metal 114. Asdiscussed with respect to FIGS. 2A-2I and FIGS. 6A-6C, the color-codedimage displayed on the display 130 is a color image in effective atomicnumber space, where each color represents a particular effective atomicnumber or a range of effective atomic numbers. Displaying the image ineffective atomic number space allows a particular item that includes aparticular material to be displayed without interference from nearbyoverlaying or underlying items made from different materials havingdistinct effective atomic numbers.

In the example shown in FIG. 1 the suitcase 110 includes a plastic pipebomb 112 filed with a plastic explosive covered by a sheet of metal 114.The suitcase 110 is placed on a conveyor belt 140 and moved into thescreener 120. In the screener 120, the suitcase 110 is exposed to x-rayradiation having at least two energies. An image of the effective atomicnumber of the suitcase 110 and the items within the suitcase 110 isdisplayed on the display 130. The settings of the display 130 are suchthat materials having effective atomic numbers similar to that of theplastic pipe bomb 112 are shown on the display 130 and items havingeffective atomic numbers similar to that of the sheet of metal 114 arenot shown. In other examples, the settings of the display 130 may besuch that the plastic pipe bomb 112 and the sheet of metal 114 areshown. The portions of the plastic pipe bomb 112 and the sheet of metal114 that overlap may be color-coded to correspond to an effective atomicnumber of interest.

Although the example shown in FIG. 1 relates to the screening of thesuitcase 110, in other examples, a system similar to the system 100 maybe used to screen items of interest other than the suitcase 110. Forexample, items of interest that are larger than the suitcase 110, suchas shipping containers carried on cargo ships and/or trains, trucks,and/or automobiles, may be scanned. In these examples, the screener 120may be made larger in order to accommodate larger sized objects. Inother examples, smaller objects, such as purses, packages, parcels,and/or briefcases, may be screened. The example system 100 discussedwith respect to FIG. 1 may be used, for example, at an airport. In otherexamples, threat detection systems similar to the system 100 may be usedat seaports, border crossings, and/or public gathering places.

Additionally, although the system 100 may be used as a threat detectionsystem, in some implementations, the system 100 may be used to analyzematerials for contaminants and/or inhomogenities. For example,contaminants and/or inhomogenities may have different effective atomicnumbers than the materials in which the contaminants and/orinhomogenities are included. Thus, the contaminants and/orinhomogenities may be identified through the effective atomic numberdisplay 130.

Referring to FIGS. 2A and 2B, an example interface 200 is shown. Theexample interface may be shown on a display configured to display acolor-coded image of an object of interest in effective atomic numberspace, such as the display 130 discussed above with respect to FIG. 1.In some implementations, the interface 200 is presented to an operatorof a threat detection system, and the interface 200 assists the operatorin determining whether objects examined by the threat detection systeminclude hazardous, or potentially hazardous, materials. The interface200 may be used to view an image generated from data received from ascreener, such as the screener 120 discussed above with respect toFIG. 1. In the examples shown in FIGS. 2A and 2B, an image 205 showsitems included in the suitcase 110. In particular, FIG. 2A shows theimage 205 with a first window of effective atomic numbers selected forinclusion in the image 205, and FIG. 2B shows the image 205 with asecond window of effective atomic numbers selected for inclusion in theimage 205.

The image 205 is a color-coded true effective atomic number image thatrepresents the effective atomic number for materials shown in the image205 with a particular color according to a legend 210. For example,materials having an atomic number of five to ten are colored orange asshown in portion 212 of the legend 210. A selector 215 indicates awindow 220 of effective atomic numbers that are imaged in the image 205.The selector 215 corresponds with the effective atomic numbers shown onthe legend 210, and the selector 215 may be placed near the legend 210for the convenience of the operator. In the example shown in FIG. 2A,the selector 215 indicates that the window 220 of effective atomicnumbers to include in the image 205 is seven to fifteen. As a result,the image 205 includes the plastic pipe bomb 112 enclosed in thesuitcase 110 discussed in FIG. 1. However, the image 205 does notinclude the sheet of metal 114, which covers the plastic pipe bomb 112,because the effective atomic number of the sheet of metal 114 is greaterthan fifteen.

A size of the selector 215 determines a range of effective atomicnumbers to include in the image 205, and a position of the selector 215determines a mean effective atomic number of the range of effectiveatomic numbers. For example, making the selector 215 larger selects alarger range of effective atomic numbers for inclusion in the image 205,and making the selector 215 smaller selects a smaller range of effectiveatomic numbers for inclusion in the image 205. In some implementations,the selector 215 may be made larger or smaller by increasing ordecreasing, respectively, the length of the selector 215. For example, aside 216 of the selector 215 may be highlighted with a mouse, or otherinput device, and dragged to make the selector 215 longer. In someimplementations, the image 205 may be displayed on a touch screendevice, and the size of the selector 215 may be increased or decreasedby the operator touching the selector 215 and manually re-sizing theselector 215. In some implementations, the selector 215 may be re-sizedautomatically based on data received from an automated process or amachine rather than from input received from an operator.

A position of the selector 215 determines the mean of the range ofeffective atomic numbers included in the window 220. For example, thecenter of the selector 215 may be located at a mid-point of the window220 with respect to the legend 210. The mid-point of the window 220 maybe the mean value of the effective atomic numbers included in the window220. In the example shown in FIG. 1, the center of the selector 215 islocated at an atomic number of eleven according to the legend 210. Theselector 215 may be re-positioned by, for example, placing a mousepointer, or some other input device, at the center of the selector 215and dragging the entire selector 215 such that the center of theselector 215 moves to a desired effective atomic number. In someimplementations, the image 205 may be displayed on a touch screen. Inthese implementations, the selector 215 may be repositioned by theoperator touching the screen where the selector 215 is shown anddragging the selector 215 such that the selector 215 is centered on theeffective atomic number of interest according to the legend 210. In someimplementations, the selector 215 may be re-positioned automaticallybased on data received from an automated process or a machine ratherthan from input received from an operator.

The interface 200 also includes bands 218 a, 218 b, 218 c, and 218 dthat indicate ranges of effective atomic numbers included in the image205 regardless of whether the ranges are selected by, for example, theselector tool 215. The effective atomic numbers included in the bands218 a, 218 b, 218 c, and 218 d may be effective atomic numbers of a setof explosives, or of a product of interest. Although four bands areshown in the examples of FIGS. 2A and 2B, other examples may includemore or fewer bands.

Referring to FIG. 2B, the image 205 may updated in response to a changein the window 220 of effective atomic numbers selected to be included inthe image 205. In particular, the change in the window 120 results inmaterials having higher effective atomic numbers being included in theimage 205 and materials having lower effective atomic numbers beingexcluded from the image 205. Thus, the sheet of metal 114 is shown inthe image 205 after the updating of the window 120, but the plastic pipebomb 112 is not shown.

The window 120 of effective atomic numbers is changed by re-sizing andrepositioning the selector 215. In particular, as compared to FIG. 2A,the selector 215 shown in FIG. 2B has been re-sized and repositionedsuch that the image 205 includes representations of materials havingeffective atomic numbers between fifteen and nineteen. Additionally, anadditional selector 215 a has been added. The additional selector 215 aallows selection of a second window 120 of effective atomic numbers forinclusion in the image 205. In the example shown in FIG. 2B, theadditional selector 215 a indicates that materials having effectiveatomic numbers between twenty-two and twenty-four are also to beincluded in the image 205. In this example, the piece of metal 114 isshown colored in a gray gradient. Although two selectors 215 and 215 aare shown in this example, in other examples more than two selectors maybe used. Using multiple selectors allows finer selection of effectiveatomic numbers, which may allow an operator to visualize materialshaving effective atomic numbers in several particular narrow bands ofeffective atomic numbers of interest that correspond to different typesof hazardous, or potentially hazardous materials.

Referring again to FIG. 2A, in some implementations, the interface 200may include selectable controls 225, 230, and 235 that allow certaintypes of materials to be included in the image 205. In the exampleshown, selection of the control “BRING ORGANICS FORWARD” 225 bringsorganic materials to the foreground of the image 205, selection of thecontrol “BRING METALS FORWARD” 230 brings metal materials to theforeground of the image 205, and selection of the control “BRINGINORGANICS FORWARD” 235 brings inorganic materials to the foreground ofthe image 205. In the example shown, the control 230 is selected inorder to bring metal materials to the foreground of the image 205. As aresult of the selection of the control 230 and/or the resizing of theselector 215, the image 205 is updated to show the sheet of metal 112 inthe foreground, as shown in FIG. 2B. The controls 225, 230, and 235 maybe selected by an operator using an input device, such as a mouse,stylus, or touch screen. In some implementations, the controls 225, 230,and 235 may be selected from the output of an automated process or amachine.

Referring to FIGS. 2C-2E, an illustration of an image of items that arecolor-coded based on effective atomic numbers of the items is shown. Anitem 242 has an effective atomic number that is color-coded as green, anitem 244 has an effective atomic number that is color-coded as orange,and an item 246 has an effective atomic number that is color-coded asblue. The items 242, 244, and 246 have different effective atomicnumbers. For example, the item 242 may be composed of an inorganicmaterial with an effective atomic number of nine, the item 244 may becomposed of an organic material with an effective atomic number ofseven, and the item 246 may be composed of a metal material with aneffective atomic number of twenty-two. The item 242 overlaps the item244 at an overlap region 247, and the item 244 overlaps the item 246 atan overlap region 248. Rather than displaying a color representing anaverage or a composite of the effective atomic numbers (such as theaverage atomic number) associated with the items 242 and 244 in theoverlap region 247, the color associated with effective atomic number ofthe item having an effective atomic number of interest is shown in theoverlap region 247. As discussed above, an effective atomic number ofinterest, or a range of effective atomic numbers of interest, may be,for example, selected by an operator or may be pre-configured values.

Referring to FIG. 2C, the item 242 has an effective atomic number thatmatches or is within a range of effective atomic numbers of interest. Asa result, the regions of overlap 247 and 248 are color-coded orange. Asa result, the entire item 244 may be discerned, which may help identifythe item 244. Referring to FIG. 2D, the item 242 has an effective atomicnumber of interest that matches an effective atomic number of interestor is within a range of effective atomic numbers of interest. The regionof overlap 247 is also color-coded green. Referring to FIG. 2E, the item246 has an effective atomic number that matches an effective atomicnumber of interest or is within a range of effective atomic numbers ofinterest, and the region of overlap 248 is color-coded to match the item246.

Referring to FIGS. 2F and 2G, another illustration of an image of itemsthat are color-coded based on effective atomic numbers of the items isshown. In this example, items 252 and 256 both have effective atomicnumbers that are color-coded green. For example, the items 252 and 256may both be inorganic materials. The item 254 has a different effectiveatomic number than items 252 and 256. The item 254 may be, for example,an organic material, and is this example, the item 254 is color-codedorange. The item 252 and the item 254 overlap at a region of overlap257, and the item 254 and the item 256 overlap at a region of overlap258. In the example shown in FIG. 2F, the effective atomic number ofinterest includes the effective number of the item 254, and the regionsof overlap 257 and 258 are color-coded orange to match the item 254. Inthe example shown in FIG. 2G, the effective atomic number of interestincludes the effective atomic numbers of the item 252 and the item 256,and the regions of overlap 257 and 258 are both color-coded green tomatch the items 252 and 256.

Referring to FIG. 2I, an illustration of an image of items that arecolor-coded based on default settings for effective atomic number isshown. The default settings indicate a preferential order in which todisplay items according to an effective atomic number associated withthe items. In the example shown, organic materials with a lowereffective atomic number (such as an item 264) have preference overinorganic materials with a mid-range effective atomic number (such as anitem 262) and metallic materials with a higher effective atomic number(such as an item 266). In this example, the item 262 is color-codedgreen, the item 264 is color-coded orange, and the item 266 iscolor-coded blue. The inorganic item 262 and the organic item 264overlap at a region of overlap 267, and the organic item 264 and themetallic item 266 overlap at a region of overlap 266. Thus, because thedefault settings specify that organic materials have a preference overinorganic materials and metallic materials, the regions of overlap 267and 268 are color-coded the same as the organic item 264. This is incontrast to techniques that show an average or composite of the atomicnumbers of the items 262 and 264 in the region of overlap 267, and anoverage or composite of the items 264 and 266 in the region of overlap268. In some implementations, an automated process may be weightedtowards organics such that organic items, such as the organic item 264are colored regardless of the composition of the background in which theitem is located. As a result, the image shows the entire organic item264, which may aid in the identification of the organic item 264.Although in the example shown, the default settings specify that organicmaterials have a preference over inorganic and metallic materials, inother examples, different orderings are possible.

Referring to FIG. 3, an example effective atomic number imaging system300 is shown. The system 300 may be similar to the system 100 discussedabove with respect to FIG. 1. The system 300 includes a screening system320 and a display 330 for imaging effective atomic numbers of screenedmaterials. The screening system 320 may be similar to the screener 320discussed and the display 330 may be similar to the display 130, both ofwhich are discussed above with respect to FIG. 1.

The screening system 320 includes an x-ray source 322, a first detector350 and a second detector 355. The screening system 320 also may includea collimator 326 and a filter 328. The x-ray source 322 exposes anobject of interest 340 to x-ray radiation of at least two energy levels.The x-rays may be collimated by the collimator 326, which may be made oflead or another material of sufficient thickness to block the x-rays.The collimated x-rays pass through the object 340, are attenuated by theobject 340 and the contents of the object 340, and the attenuated x-raysare sensed by the first detector 350. The first detector 350 may be, forexample, a scintillator, and the x-rays may pass through the firstdetector 350. The filter 328 may be placed in front of the seconddetector 355 such that only x-rays having energies below a cut-offenergy of the filter 328 reach the second detector 355. The filter 328may be made from a metal material such as, for example, copper. Thearrangement of the first and second detectors shown in the example ofFIG. 3 may be referred to as a front-to-back configuration. In afront-to-back configuration, the detectors 350 and 355 image the samearea, thus data collected by the detectors 350 and 350 generally isaligned at the time of detection without further correction. In someimplementations, the first detector 350 and the second detector 355 maybe placed next to each other in a side-by-side configuration.

Thus, the first and second detectors sense attenuated x-rays ofdifferent energy levels, with the second detector sensing theattenuation of the higher energy x-rays. The attenuation of the lowerand higher energy x-rays may be compared such that the effective atomicnumber of the material of the suitcase and the materials of items withinthe object 340 is determined. For example, an attenuation measurementmay be taken for each pixel (which corresponds to a portion of theobject 340) to determine how much attenuation of the x-rays the object340 caused. The attenuation values may be determined from a ratio of theintensity of the detected x-ray energy to the intensity of the initialx-ray energy from the x-ray source 322. Attenuation pairs (e.g.,attenuation of the higher energy x-ray and attenuation of the lowerenergy x-ray) are determined for the pixels over the exposed region ofthe object 340. The attenuation pairs are compared to attenuation valuesof nearby pixels to determine whether differences in attenuation betweennearby pixels is attributable to the presence of a specific material. Inthis manner, an estimate of the effective atomic number of the materialmay be determined, even if the material is overlayed or underlayed bymaterials having a different atomic number. In the example shown in FIG.3, a plastic pipe bomb 312 is overlayed by a sheet of metal 314. Forexample, the plastic pipe bomb 312 may overlay the sheet of metal 314 ifthe plastic pipe bomb 312 is further from the first detector 350 thanthe sheet of metal 314. The effective atomic number may be contrasted tothe average atomic number. Continuing the above example, the averageatomic number is the average of the effective atomic number of thematerial of the object 340 and all of the materials of all of the itemsincluded in the object 340. In comparison, the effective atomic numberis the atomic number of each of the materials within the object 340, andthe effective atomic number is a unique characteristic of each material.

The system 300 also includes the display 330, which displays theeffective atomic number determined as discussed above. The display 330also displays an interface that allows the effective atomic numbersincluded in the displayed image to be varied, such as the interface 200discussed above with respect to FIGS. 2A and 2B. In the example shown,the effective atomic numbers included in the displayed image include aneffective atomic number of the plastic pipe bomb 314. Thus, the plasticpipe bomb 314 is shown on the display 330.

Referring to FIG. 4, a block diagram of an example effective atomicnumber imaging system 400 is shown. The system 400 includes a screeningsystem 410 and a display system 450 that receives and displays data fromthe screening system 410 as an image in effective atomic number space.

The screening system 410 may be used to screen objects to determinewhether the object includes materials of interest. In particular, thescreening system 410 determines an effective atomic number of materialson an in the object and outputs the effective atomic number of thematerials to the display system 430. The screening system 410 includes areceiving region 415 that is configured to receive an object to bescreened, an imaging system 420, an effective atomic numberdetermination module 430, a processor 435, and an input/output device440.

The receiving region 415 is appropriately sized depending on the objectto be screened. For example, the receiving region 415 may be largeenough to receive a suitcase or other luggage item. In other examples,the receiving region 415 may accommodate a truck or shipping container.In other examples, the screening system 410 may be used to detectinhomogeneities and/or contaminants in, for example, processed fooditems such as cereals. In this example, the receiving region 415 may besized to fit around a conveyor belt in a food processing plant.

The screening system 410 also includes the imaging system 420, whichincludes a source 422 and a sensing module 424. The source 422 may be asource that emits x-rays of at least two energies, and the source 422may be similar to the x-ray source 322 discussed above with respect toFIG. 3. The sensing module 424 includes detectors to sense x-rays thatare produced by the source 422. The sensing module 424 may include afilter such that the detectors included in the sensing module 424 senseeither higher energy x-rays or lower energy x-rays. The effective atomicnumber determination module 430 determines the effective atomic numberof the materials through which the x-rays generated by the x-ray source422 pass. The module 430 may use the detected attenuated x-rays todetermine the effective atomic numbers as discussed above with respectto FIG. 3. In the example shown in FIG. 4, the effective atomic numberdetermination module 430 is shown as a component of the screening system410, this is not necessarily the case. In other examples, the module 430may be implemented as component separate from, and in communicationwith, the screening system 410.

The screening system 410 also includes the processor 435, theinput/output device 440, and the memory 445. The memory 445 storesinstructions that, when executed by the processor 435, cause theeffective atomic number module 430 to perform operations such asdetermining the effective atomic number of materials include in theobject of interest based on the sensed attenuated x-rays. The memory 445also may store data sensed by the sensor module 420 and instructions forretrieving the data from the sensor module 420. The memory 445 may beany type of computer-readable medium. The processor 435 may be aprocessor suitable for the execution of a computer program such as ageneral or special purpose microprocessor, and any one or moreprocessors of any kind of digital computer. Generally, a processorreceives instructions and data from a read-only memory or a randomaccess memory or both. The processor 435 receives instruction and datafrom the components of the screening system 410, such as, for example,the imaging system 420 and/or the effective atomic number determinationmodule 430, to, for example, analyze data from the imaging system 420 todetermine effective atomic numbers. In some implementations, thescreening system 410 includes more than one processor. The input/outputdevice 445 may be any device able to transmit data to, and receive datafrom, the screening system 410. For example, the input/output device 445may be a mouse, a touch screen, a stylus, a keyboard, or any otherdevice that enables a user to interact with the screening system 410. Insome implementations, the input/output device 445 may be configured toreceive an input from an automated process or a machine.

The display system 450 includes an input module 455, an interfacegeneration module 460, a processor 470, and an input/output device 480.The input module 455 receives effective atomic number information fromthe screening system 410 or the effective atomic number determinationmodule 420. The interface generation module 460 displays an image ineffective atomic number space of the object screened by the screeningsystem 410. The interface generation module 460 also generates anddisplays an interface that allows interaction with the displayed image.The interface may be similar to the interface discussed above withrespect to FIGS. 2A and 2B.

The display system 450 also includes a processor 470 and an input/outputdevice 480. The processor 470 executes instructions that cause theinterface generation module 460 to generate and display the interfaceand process commands received from the input/output device 480. Theinput/output device 480 may be any device that allows a user to interactwith the display system 450. For example, the input/output device 480may be a mouse, a keyboard, or a touch screen.

Referring to FIG. 5, an example process 500 for imaging effective atomicnumbers of materials is shown. The process 500 may be performed by, forexample, one or more processors included in a system such as the system400 discussed above with respect to FIG. 4. In some implementations, theprocess 500 may be performed by the display system 450.

Effective atomic numbers associated with pixels in a region are received(510). An effective atomic number is associated with a pixel in theregion. In some implementations, the region is a representation of anarea imaged by an imaging system. For example, the region may representan image of a physical area that includes an object of interest, such asa suitcase that may contain explosives. In other examples, the regionmay be a portion of a physical area that includes an object of interest.

In implementations in which the region is exposed to x-rays of twoenergies, a higher-energy x-ray and a lower-energy x-ray, the effectiveatomic numbers associated with the pixels in the region may bedetermined as follows. The attenuation of the lower and higher energyx-rays may be compared such that the effective atomic number of thematerial of the suitcase and the materials of items within the object ofinterest is determined. For example, an attenuation measurement may betaken for each pixel (which images a portion of the physical area thatincludes the object of interest) to determine how much attenuation ofthe x-rays the object of interest caused. The attenuation values may bedetermined from a ratio of the intensity of the detected x-ray energy tothe intensity of the initial x-ray energy from the x-ray source.Attenuation pairs (e.g., attenuation of the higher energy x-ray andattenuation of the lower energy x-ray) are determined for the pixels inthe region. The attenuation pairs are compared to attenuation values ofnearby pixels to determine whether differences in attenuation betweennearby pixels is attributable to the presence of a specific material. Inthis manner, an estimate of the effective atomic number of the materialmay be determined for each pixel, even if the material is overlayed orunderlayed by materials having a different atomic number.

X-ray data is received for the region (520). The x-ray data may be thehigher-energy x-ray energy and/or the lower-energy x-ray energy. An itemwithin the region is identified using the x-ray data (530). Continuingthe example above, the item may be an image of an item in the suitcase.For example, the item may be a metal weapon or a plastic object. Theitem may be identified using the x-ray data by, for example, applying anedge-detection algorithm to the data to outline edges of items imaged inthe x-ray data. In some implementations, the item may be identified byaggregating pixels having similar or the same effective atomic numberinto clusters and identifying boundaries between clusters as boundariesof the item. Some of the pixels in the region are correlated with theitem such that the item is associated with an effective atomic number(540). The pixels in the region that are located within or on theboundary of the item are correlated with the item. Because the pixelsare each associated with an effective atomic energy number, the item isassociated with the effective atomic energy number of the pixels. Insome examples, the item may be composed of more than one material, andthe materials may have different effective atomic numbers. For example,a homemade bomb may include plastic explosives and a metal detonator,and the plastic explosives and the metal detonator have differenteffective atomic numbers. Thus, in these examples, the item includespixels having different effective atomic numbers, and the item is notassociated with one effective atomic number.

An image of the item is rendered (550). In the rendered image, the itemhas a display style that is based on the effective atomic number of theitem. For example, the display style may be color, and, in theseexamples, the rendered image is a color-coded image of the item. Forexample, a metallic item may have a display style of blue in acolor-coded image. In other examples, the display style may be apattern, such as shading or cross-hatching, or the display style may bea texture. In some examples, the display style may be more than one ofcolor, texture, and a pattern. In examples in which the display style isa pattern, a metallic item may, for example, have a display style ofdots. Some examples may have a display style that combines color andpatterns. In some examples, a second item may be identified, and a rangeof effective atomic numbers of interest may be received. The second itemmay overlap, or cover the identified item. For example, the identifieditem may be a plastic item, and the second item may be a piece of sheetmetal partially covering the plastic item. The overlapping area wherethe second item covers the plastic item may be referred to as the regionof overlap. Rather than being represented as an average or composite ofthe effective atomic numbers of the plastic item and the sheet metal,the region of overlap may have the display style of either the displaystyle of the plastic item or the display style of the sheet metal. Inparticular, the region of overlap has the display style of the sheetmetal if the effective atomic number of the sheet metal is in the rangeof effective atomic numbers of interest, and the region of overlap hasthe display style of the plastic item if the effective atomic number ofthe plastic item is within the range of effective atomic numbers ofinterest. In some implementations, the display style may be a texture,cross-hatching, and/or shading in addition to, or instead of, color.

Referring to FIG. 6A, an example image 600A shows an image of a suitcase610 and the contents of the suitcase 610, including a metal gun 620 anda plastic item 630 that is constructed from metal and cloth andenclosing metal materials. The image 600A is in effective atomic numberspace and is color-coded according to a legend 640. The image 600Aincludes those effective atomic numbers that are included in a rangespecified by a selector 650. In the example shown in FIG. 6A, theselector 650 indicates that atomic numbers five to twenty-three areincluded in the image 600A. A selection of a range of effective atomicnumbers of interest may be received based on the size and/or position ofthe selector 650.

Referring to FIG. 6B, the selector 650 is re-sized and re-positionedsuch that the window of atomic numbers of interest is between 5 andabout 10.5. Thus, a window of effective atomic numbers between 5 andabout 10.5 is received in this example. The color coded-image of thematerials associated with the object of interest is updated such thatthe updated image includes the selected effective atomic numbers ofinterest. Continuing with the example shown in FIG. 6B, the image 600Ais updated to an image 600B that includes effective atomic numbersbetween 5 and 10.5. As a result, the metal objects, such as the gun 620,which have effective atomic numbers beyond the selected range, arede-emphasized. The non-metallic objects, such as the item 630, becomemore prominent. Referring to FIG. 6C, the selector 650 is re-sized andrepositioned such that the selector 650 indicates that effective atomicnumbers between 16 and 23 are included in an updated image 610C. In someimplementations, a histogram (not shown) of effective atomic numbersincluded in the image may be displayed. The histogram represents thepercentage of pixels in the image at each of the effective atomicnumbers shown in the legend 640.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the scope of the disclosure. Accordingly, other implementations arewithin the scope of the following claims.

1. A method of imaging effective atomic numbers, the method comprising: receiving effective atomic numbers associated with pixels in a region, wherein an effective atomic number is associated with each pixel in the region; receiving x-ray data for the region; identifying an item within the region from the x-ray data; correlating some of the pixels in the region with the item such that the item is associated with an effective atomic number; and rendering an image of the region, the pixels of the item having a display style based on the effective atomic number of the item.
 2. The method of claim 1, further comprising receiving an indication of a range of effective atomic numbers of interest.
 3. The method of claim 2, wherein the display style comprises color, and a color is associated with the range of effective atomic numbers of interest.
 4. The method of claim 3, further comprising identifying additional items within the region from the x-ray data, the additional items covering the item in a region of overlap, and the additional items have effective atomic numbers different than the effective atomic number of the item.
 5. The method of claim 4, wherein the effective atomic numbers of the additional items are within the range of effective atomic numbers of interest, and the additional items and the region of overlap are represented by the color associated with the range of effective atomic numbers of interest.
 6. The method of claim 2, wherein the display style comprises one or more patterns, and a pattern is associated with the range of effective atomic numbers of interest.
 7. The method of claim 2, wherein receiving an indication of a range of effective atomic numbers of interest comprises accessing a predetermined range of effective atomic numbers of interest.
 8. The method of claim 1, further comprising displaying a re-sizeable selector that allows selection of one or more ranges of effective atomic numbers of interest.
 9. The method of claim 8, wherein the selector is movable.
 10. The method of claim 9, wherein a size of the selector corresponds to one or more ranges of effective atomic numbers of interest, and a position of the selector corresponds to a mean of the effective atomic numbers included in the range.
 11. The method of claim 1, wherein identifying an item within the region from the x-ray data comprises determining edges within the x-ray data and identifying the item based on the edges.
 12. The method of claim 1, wherein identifying an item within the region from the x-ray data comprises aggregating pixels having similar attenuation values and identifying an item based on one or more boundaries around the aggregated pixels.
 13. A system of imaging effective atomic numbers, the system comprising: a processor configured to: receive effective atomic numbers associated with pixels in a region, wherein an effective atomic number is associated with each pixel in the region, receive x-ray data for the region, identify an item within the region from the x-ray data, correlate some of the pixels in the region with the item such that the item is associated with an effective atomic number; and a display configured to render an image of the region, the pixels of the item having a display style based on the effective atomic number of the item.
 14. The system of claim 13 further comprising a source configured to expose an area imaged by the pixels to x-rays of at least two energies.
 15. The system of claim 14 further comprising a sensor configured to detect x-rays of at least two energies.
 16. The system of claim 15 further comprising a filter positioned between the area and the sensor, the filter configured to separate the x-rays of at least two energies.
 17. The system of claim 13, wherein the display is a touch screen.
 18. The system of claim 13, further comprising an opening configured to receive an object.
 19. The system of claim 13, wherein the opening is configured to receive cargo and the object comprises luggage.
 20. A method of imaging effective atomic numbers, the method comprising: receiving effective atomic numbers associated with pixels in a region, wherein an effective atomic number is associated with each pixel in the region; receiving penetrating spectral data for the region; identifying an item within the region from the penetrating spectral data; correlating some of the pixels in the region with the item such that the item is associated with an effective atomic number; and rendering an image of the region, the pixels of the item having a display style based on the effective atomic number of the item.
 21. The method of claim 20, wherein the penetrating spectral data comprises data based on x-rays of at least two energy levels.
 22. A computer program tangibly embodied on a computer-readable medium, the computer program including instructions that, when executed, cause a processor to perform operations comprising: receiving effective atomic numbers associated with pixels in a region, wherein an effective atomic number is associated with each pixel in the region; receiving x-ray data for the region; identifying an item within the region from the x-ray data; correlating some of the pixels in the region with the item such that the item is associated with an effective atomic number; and rendering an image of the region, the pixels of the item having a display style based on the effective atomic number of the item. 